NOTE: To observe the relationship between form and function in animals, you will be examining and dissecting a cricket and a crayfish. Each pair of students will receive one of each of these organisms. When dissecting, be sure to wear gloves, closed-toed shoes, and safety goggles. Take care when handling dissecting tools, as they may be sharp. HYPOTHESES: In this experiment, the experimental hypothesis could be that the cricket and crayfish have similar body structures because they are both arthropods, but that they will have some specific adaptations for their different habitats and lifestyles. The null hypothesis might be that the cricket and crayfish have identical adaptations and body plans.
To begin, place the cricket on its back and observe its appendages.
In Table 1, make detailed notes about your observations and try to predict the potential function of each of the forms you observe. Click Here to download Table 1
Do the same with the wings of the cricket if they are present.
Consider how the structure of the wings benefits the cricket.
If the cricket does not have wings, consider the reasons behind their absence.
If the cricket is female, it will have an ovipositor. This can be observed as a long dark tube protruding from the posterior end and it is used to deposit eggs.
Consider why the ovipositor is useful.
Next, flip the cricket over onto its back and observe the mouth parts under a microscope. NOTE: Use a probe to move structures when necessary.
Make a hypothesis about the size and type of prey the cricket consumes based upon your observations.
Continue to examine the external parts of the cricket, making hypotheses about their function until you have filled at least five rows in the table with your observations.
Using tweezers to hold the cricket down, carefully remove the wings and legs from the body with the scissors.
Next, position the body of the cricket so that its underside is facing down.
Again with the scissors, cut the thin outer exoskeleton from the anus to the head trying not to damage any of the organs underneath.
Use the tweezers to open the incision and dissecting pins to hold open the exoskeleton.
Next, use the water pitcher to carefully add water to the Petri dish until it reaches just below the top edge. This should make the exposed organs of the cricket easier to see.
Move the Petri dish to the stage of the dissecting microscope and turn on the light.
Use the cricket figure as a reference and observe the organs under the dissecting microscope. Where necessary, use the probe to move organs that may be blocking others.
Identify as many internal structures as possible and fill out the form and predicted function for at least three internal structures.
Next, you'll observe the outer and inner structures of the crayfish. Begin with the crayfish on its back and observe the appendages.
In Table 1, take notes about the structures you notice.
Observe the tail. Devise a hypothesis for its jointed structure. Consider how a crayfish moves in its environment.
Next, with the crayfish still on its back, look for the swimmerets under its tail. NOTE: If the crayfish is male, the swimmerets nearest the legs are long extending between the legs. These are specialized reproductive structures that transfer sperm to the female. Females do not have these long structures, but instead have swimmerets that store developing embryos.
Consider the benefits of this strategy. Be sure to observe both a male and female crayfish, checking with neighboring students to identify these structures.
Next, observe the mouth parts of your crayfish using a probe.
Based on these structures, make a hypothesis about the size and type of prey you think crayfish consume.
Before beginning the dissection, be sure to fill in Table 1 with at least five external structures you observed in the crayfish along with their predicted function. With the organism lying legs down, use scissors to cut an incision in the exoskeleton from the tail to between the eyes. Take care to not disturb the organs underneath.
Next, remove the carapace as demonstrated in the video.
Open the tail and use the pins to secure the exoskeleton down to the dissecting tray exposing the organs.
Observe the organs first with the naked eye. Use the figure to identify the different body parts of the animal.
Then, if there is enough space on the stage, place the dissecting tray with the crayfish under the dissecting microscope to view these structures more closely.
Fill out at least three rows of Table 1 with these observations and predicted functions of internal structures in the crayfish.